Heat is often created as a byproduct of industrial processes and is discharged when liquids, solids, and/or gasses that contain such heat are exhausted into the environment or otherwise removed from the process. This heat removal may be necessary to avoid exceeding safe and efficient operating temperatures in the industrial process equipment or may be inherent as exhaust in open cycles. If the heat is not recovered from this exhaust, however, it represents inefficiency, as thermal energy is lost. Accordingly, industrial processes often use heat exchanging devices to recover the heat and recycle much of it back into the process or provide combined cycles, utilizing this heat to power secondary heat engines.
Such recovery can be significantly limited by a variety of factors. For example, the exhaust stream may be reduced to low-grade (e.g., low-temperature) heat, from which economical energy extraction is difficult, or the heat may otherwise be difficult to recover. Accordingly, the unrecovered heat is discharged as “waste heat”—typically via a stack or through exchange with water or another cooling medium. In other settings, heat is available from renewable sources of thermal energy, such as heat from the sun or geothermal sources, which may be concentrated or otherwise manipulated. Heat provided by these and other thermal energy sources is also intended to fall within the definition of “waste heat”—as that term is used herein.
Such waste heat can be utilized by turbine generator systems arranged as part of thermodynamic cycles, such as the Rankine cycle, to convert heat into work. Furthermore, supercritical carbon dioxide power cycles may be useful in applications where conditions are not conducive to the use of more-conventional working fluids and/or to increase efficiency when compared to such working fluids. Supercritical carbon dioxide working fluid provides numerous thermodynamic advantages. For example, by using a supercritical fluid, the temperature glide of a process heat exchanger can be more readily matched.
In these cycles, it is important to manage the low-side pressure upstream from the system pump. If the low-side pressure drops below the saturation point of the working fluid, cavitation can occur, which can damage the pump. On the other hand, the pressure ratio between the low side and the high side is directly related to the power generation of the system, with efficiency and power generation being highly sensitive to changes in the low-side pressure, even as compared to the high-side.
Accordingly, it is desirable to maintain the lowest possible, safe low-side pressure. In the past, systems of vents, pressure containment vessels, and other equipment have been used as mass management systems, with a good degree of success, to maintain desired operation parameters. However, these systems often allow pressure to be vented to the system on nearly a constant basis. This represents wasted working fluid, which must be replenished on a periodic basis, thereby increasing operating costs.
Therefore, there is a needed for a heat engine system, a mass management system, a method for regulating pressure in the heat engine system, and a method for generating electricity, whereby the systems and methods provide maintaining a desired range of pressure within the system, avoiding on-going ventilation of the process fluid, and maximizing the efficiency of the heat engine system to generate electricity.